1. Field of the Invention
This invention relates to thermal cracking of a gas oil. It particularly relates to a mild thermal cracking of gas oil to minimize the production of naphtha and increase the amount of middle distillate components.
2. Description of the Prior Art
Mild thermal cracking processes are well known in the art. Heavy hydrocarbonaceous oils are subjected to mild thermal cracking processes to convert at least a portion of the high boiling components to lower boiling components. The naphtha component which is obtained from thermal cracking processes is usually of poor quality, that is, it has a high content of olefins and sulfur which makes it undesirable for many end uses. Improvements in thermal cracking have been proposed to minimize the amount of naphtha product, that is, to increase the amount of middle distillate components relative to the amount of naphtha component obtained from the process. For example, Netherland patent application No. 282,794 discloses a mild thermal cracking process (visbreaking) of heavy hydrocarbon oils containing residual fractions in which the process is conducted in the presence of added naphtha, such as a recycle naphtha stream from the visbreaking process, to decrease the net naphtha yield and to increase the middle distillate component. The term "net yield" relative to naphtha refers to the total naphtha yield less the quantity of added naphtha.
It is also known to recycle countercurrently the overhead products including and up to light gas oil to a visbreaking stage of heavy hydrocarbon oils, such as reduced crude. The recycled overhead products provide the endothermic heat of conversion of the heavy oil and the heat of vaporization of the distillate material (see, for example, British Pat. No. 722,369).
A vapor phase type cracking of heavy oil, which may be a gas oil, is known in which an aromatic product is recycled. The final product includes an increased amount of butadiene and ethylene (see U.S. Pat. No. 2,378,067).
A one or two-step process is known for the thermal treatment of heavy hydrocarbons boiling mostly above 1000.degree. F. The process includes a recycle step wherein a portion of low boiling material, which may have a boiling range below 650.degree. F., and all of the heavy material including unconverted feed, is recycled to the thermal cracking step (see U.S. Pat. No. 3,707,459).
A catalytic cracking process is known in which a gas oil is cracked in the presence of the catalyst and added naphtha to obtain an increased yield of middle distillate (see U.S. Pat. No. 3,954,600).
It has now been found that thermal cracking of a feedstock consisting essentially of gas oil in the absence of a catalyst and in the presence of added naphtha only will minimize the net yield of naphtha in the thermally cracked product.